Capturing CO2 from power plants that use fossil fuels is one of several strategies to reduce global CO2 emissions. The task of removing CO2 from power plant flue gas is challenging because existing methods for separate CO2 from the gas mixture requires a significant portion of power plant output. The separation task can be simplified by replacing conventional air with pure oxygen so that the combustion products are just CO2 and water, which may be easily separated by condensation. However, current commercial techniques for producing oxygen from air require very energy-intense cryogenic processes. Chemical looping combustion (CLC) is a novel combustion technology that utilizes an oxygen carrier, such as metal oxide, to transport oxygen from air to fuel, thereby avoiding direct contact between fuel and air. The significant advantage of CLC over conventional combustion is that CLC can produce a sequestration-ready CO2 stream—not diluted by nitrogen (N2)—without expending any major energy required for the separation of CO2. The overall CLC process, in which the metal oxide cycles between oxidized and reduced states, is exothermic. Several single metal oxides and bi-metallic oxides have been reported in the literature as oxygen carriers a promising bi-metallic oxygen carrier containing CuO and Fe2O3 for both methane and coal CLC.
Production of hydrogen from methane has received much attention because it is a promising energy source that is also environmentally benign. Hydrogen is used in oil refineries, for ammonia, methanol production, and fuel cells. Steam methane reforming (SMR) is currently the most popular commercial method of producing hydrogen. Synthesis gas produced in SMR must be further processed in the water-gas shift reactor to produce a gas stream containing H2 and CO2. An additional step is required to separate CO2 and H2 to produce pure H2 and sequestration ready CO2. The energy for the SMR process is provided via methane combustion in air which produces a CO2 stream diluted with nitrogen and will require separation prior to sequestration.
Various researchers have reported on the production of hydrogen and synthesis gas using the chemical looping methane reforming process. Methane partial oxidation using an oxygen carrier is one of the processes reported for the production of synthesis gas. In this process, an oxygen carrier is used directly in the fuel reactor to partially oxidize hydrocarbons. Another process reported in the literature for hydrogen production via CLC includes initial reduction of the oxygen carrier with fuel, such as methane or synthesis gas, followed by steam oxidation to produce hydrogen via water splitting. A combination of partial oxidation with oxygen carriers and hydrogen production via water splitting on the reduced oxygen carrier is also reported. Other approaches reported include integration of a traditional hydrocarbon steam reformer with the CLC process, and a five step process to produce synthesis gas from the CLC process using NiO as the oxygen carrier and the reduced carrier as the steam reforming catalyst. The processes described in this disclosure use neither partial oxidation of methane nor hydrogen production via water splitting using steam oxidation.
Thermo-catalytic decomposition of methane to carbon and hydrogen has received attention because the process produces hydrogen directly without any additional gas processing. A recent systems analysis indicated that the cost of hydrogen production by thermal decomposition of methane is lower than the cost for the steam reforming process. Catalysts containing nickel and iron have been widely used for methane decomposition tests. In addition, carbon formed in the methane decomposition process has also a commercial value. This disclosure describes a process for producing hydrogen and carbon by methane decomposition on copper oxide-iron oxide catalysts coupled with methane CLC using a CuO—Fe2O3 oxygen carrier. This CuO—Fe2O3 is used as the oxygen carrier for the chemical looping process while the reduced CuO— Fe2O3 carrier is used for the catalytic decomposition process to produce hydrogen. The process produces a pure hydrogen stream and carbon along with a sequestration-ready CO2 stream. In addition to pure hydrogen, steam gasification of carbon formed during methane decomposition produces a synthesis gas stream with the ratio of H2/CO of 2, which is suitable for chemical production.
The second process described in this paper occurs after the CLC process with the CuO—Fe2O3 oxygen carrier. The reduced oxygen carrier is used directly for the SM R process to produce synthesis gas, similar to the commercial steam reforming process with nickel-based catalysts. However, the heat required for the SMR process is supplied by the CLC reaction with the oxygen carrier. Syngas has many commercial applications: it can be used in the Fisher-Tropsch process to produce diesel, or converted into other useful chemicals such as methanol and dimethyl ether. Methanol is used as the feedstock for production of formaldehyde, acetic acid, propylene, and various esters, which are the chemical building blocks in the production of plastics, resins, pharmaceuticals, adhesives, paints, and much more. Nickel-based catalysts are traditionally used in the commercial steam reforming process and noble metal catalysts have also been reported. The reduced form of the CuO—Fe2O3 catalyst is environmentally benign unlike nickel catalysts, and the cost of the reduced CuO—Fe2O3 catalysts is significantly lower than noble metal catalysts used in steam reforming processes.